Resistance element made from ceramic material are used at very high operating temperatures., commonly in the interval 1200-1900° C. The operating conditions are often intermittent operation which means that the elements shift between heating and cooling off. At these temperatures the properties of the material is affected so that it becomes softer and more plastic. The changed properties have to be taken into account at the installation as they may cause changes of the shape of the elements and consequently less good function. On certain kinds of elements there may also be formed a surface layer having properties which are different from those of the body of the element material, for example there are formed in an oxidising atmosphere, for example air, a surface layer of silicon dioxide on elements made from molybdenum dioxide.
In order to reduce the undesired effects of the above described conditions elements of this kind are often mounted in special ways, for example hanging or lying on support plates which may be made from aluminium oxide. These means in themselves bring with them some disadvantages as the elements tend to adhere to the supporting means which may cause cracks and broken elements when the elements cool off. In those cases where the supporting plates are arranged with spaces between adjacent plates there is a risk for sagging of the element between the plates which further increases the risks for broken elements. It is also known to place the glow zones of the elements on a bead of sand which means that the elements are free to move when the length of them changes at changing temperature, this often means that the elements dig themselves down into the sand. The maximum element temperature at this kind of installations usually is about 1600° C.